Measuring based on a capacitive acceleration sensor has proved to have a simple principle and to provide a reliable method in the measuring of acceleration. The capacitive measuring is based on a change in the gap between two surfaces of a pair of electrodes of the sensor. The capacitance between the surfaces, i.e. the capacity for storing electric charge, depends on the area of the surfaces and on the distance between the surfaces. Capacitive measuring can be used already at rather low measuring ranges of acceleration.
The prior art is described below with exemplifying reference to the accompanying figures, of which:
FIG. 1 shows a perspective view of the structure of a pair of electrodes of an acceleration sensor according to prior art, and
FIG. 2 shows a side view of the functional structure of a translatory motion based pair of electrodes of the acceleration sensor according to prior art.
FIG. 1 shows a perspective view of the structure of a pair of electrodes of an acceleration sensor according to prior art. The pair of electrodes of the acceleration sensor according to prior art comprises a movable electrode 1, which moves according to the acceleration, and a stationary electrode 2. The movable electrode 1 is that part 1 of the acceleration sensor, which is responsive to the acceleration, and which part, in consequence of the acceleration, moves in relation to the stationary electrode 2. The movable electrode 1 and the stationary electrode 2 constitute a pair of electrodes converting acceleration into a quantity that can be measured electrically, i.e. capacitance. In the Figure, the movable electrode 1 of the acceleration sensor is supported at points 3 and 4. Generally, on the opposite side of the movable electrode 1, the acceleration sensor of prior art also comprises a second pair of electrodes, which, for clarity reasons, is not shown in the Figure.
The acceleration sensor can be implemented based on either a translatory motion or a rotational motion of the movable electrode of the pair of electrodes.
FIG. 2 shows a side view of the functional structure of a translatory motion based pair of electrodes of the acceleration sensor, according to prior art. The pair of electrodes of the acceleration sensor, according to prior art, comprises a movable electrode 1 and a stationary plate portion 2.
The support point of the movable electrode 1 of the acceleration sensor is indicated by point 4. As the movable electrode 1 of the acceleration sensor is in an upper position, a capacitance is formed between the bottom surface of the movable electrode 1 and the top surface of the plate portion 2. The magnitude of the capacitance depends on the area of the surfaces 1, 2 and the distance between the surfaces 1, 2. When the movable electrode 1 of the acceleration sensor moves to a lower position, the capacitance between the surfaces 1, 2 increases considerably, as the distance between the surfaces 1, 2 decreases.
The implementation of the acceleration sensor according to the present invention using several pairs of electrodes is described in more detail in the Applicant's co-pending international patent application.